1. Field of the Invention
The present invention relates generally to a method of forming impact and tear resistant film, and the film produced thereby. More particularly, the present invention is directed to a commercially viable process for producing a laminated film having excellent impact resistant and tear resistant properties.
2. Description of Related Art
Impact resistant safety films are commercially available for use where reinforcement is desired, particularly for uses such as protective glazings. For such applications, single-layer or multiple-layer films are adhered to glass sheets, typically on the inside portion of the glazing facing the occupants. Glazing protected with safety films include, but are not limited to, window glass, shower doors, display cases and the like. Safety films are adhered to the glass via various adhesives standard in the industry, and such application of film may be in the so-called “daylight” method in which the film is applied to the glass face without additional anchoring (with or without “catch bars”), or, preferably, the film is used in conjunction with a framing system designed to hold the film in the frame in the event of glass breakage. Such films (and associated framing systems) limit damage and injury to occupants and materials interior to the protected structure when the glass is impacted exteriorly by storm-borne debris, attempted forced penetration during robberies, or pressure waves associated with bomb blast or explosion due, for example, to terrorist activities. In glazed structures unprotected by safety films, glass shards enter the interior of the structure when impacted by debris or blasts and shower the occupants with the glass “spall” resulting in serious injury or death. In addition, if the unprotected glass is broken during a storm, water and wind damage to the interior of the structure and its contents results in significant loss of use and high replacement and repair costs. With glazing protected by safety films (either single-layer or multiple-layer (laminated) films), the spalling effect is eliminated or significantly reduced by the adherence of the shattered glass to the film. If used in conjunction with framing systems, penetration of the glazing into the interior of the structure can be eliminated or curtailed to the point that no injury to occupants or damage to property occurs. Studies have shown direct correlation between tear resistance of films and film laminates, and impact/blast resistance of the glazing structures using safety films and film laminates. Other studies have also shown that multiple-layer safety films (laminates) perform better than single-layer films of the same thickness with respect to film tear properties. Tear resistant films have a wide variety of applications, from packaging to video, and any other film application in which enhanced resistance to tearing is beneficial.
Safety films are preferably transparent, to permit them to be used with glass for glazing applications or other transparent end products. Consequently, polymer films have been found to be well-suited to this use. Safety films are often themselves laminated structures, formed of multiple layers of polymer film, whether unslit or slit. Such multiple layers typically provide greater thickness and consequently, greater impact and tear resistance, than single thickness films. However, as the number of layers increases, the clarity of the film decreases, making the film less desirable for glazing use. The cost of higher numbers of layers, and the resulting thickness, can also be limitations. Thus, safety film laminates having excellent impact resistance and tear resistance with a minimum of layers are desirable.
Cross-laminated films are known. In such films, two or more webs of film are solely or primarily uniaxially oriented. The webs are then crossed at right angles so that the primary direction of orientation of the two films is also at (or nearly at) right angles to one another. The intersecting portion of the two films can then be removed from the webs and used. However, such piecing of the web is not commercially practical. It creates short web segments that may not be useful in automated operations. It requires substantial labor to turn, laminate and cut the film. Uniaxially oriented film is also more prone to tearing and more difficult to work with than biaxially oriented film. A method of forming a laminated film that is free of one or more of these limitations is desirable.